Castle: Let me borrow your magnifying glass.
Beckett: I don't have a magnifying glass.
Castle: Isn't that standard issue for detectives?
Beckett: No, not since Sherlock Holmes.
Sherlock Holmes is the main character in a series of detective novels by author Sir Arthur Conan Doyle. A movie version was released shortly after this episode aired starring Robert Downey Jr. as the title character.